For starters, Kathleen Vohs, an associate professor of marketing at the University of Minnesota, notes that "[t]here is research that shows people still have the same self-control as in decades past, but we are bombarded more and more with temptations," and that "[o]ur psychological system is not set up to deal with all the potential immediate gratification."

The result is that we're constantly exercising willpower and self-control. Makes sense, but the problem is that willpower is like a muscle capable of fatigue: you can't keep any muscle flexed forever. To test this, researchers placed participants in situations in which they had to practice self-control—not laughing at a funny movie or not eating chocolate-chip cookies in front of them. The control group could laugh or eat as many cookies as they wanted.

The results? The group that had to resist temptation did not perform as well on the second task as the group that was allowed to give in to temptation, said Timothy A. Pychyl, an associate professor of psychology at Carleton University in Ottawa.

The conclusion was that those who had to exert more willpower in the first task "exhausted their self-regulatory strength, at least temporarily, and therefore are unable to muster the self-regulation needed for the second task."

Hit up the Times article for more, including suggestions for framing your goals more specifically.